janiie
Suspect the power supply has failed and as you need a'known' working PSU to test the computer/motherboard, it should be replaced.
Generic standard ATX power supply units can be installed, with either a 24-pin or 20+4-pin main motherboard power connector, with or without the on/off switch.
You should be able to buy a compatible power supply from either a local or online computer store.
Note: A power supply with four SATA power connectors, is required.
The power supply replacement procedures are here:
ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_inspiron_desktop/inspiron-530_owner%27s%20manual_en-us.pdf
Bev.
Ps. rdunnill has already posted a link to a 350w compatible power supply.